Cotton t-shirts have a lot going for them, they tend to be more economical and will show colors better and those colors will really pop. Cotton will also be a little thicker and slightly heavier in weight making them a bit warmer.
Tri-blends are a blend of three materials: cotton, polyester, and rayon. These shirts are much thinner and lighter in weight. They tend to be a bit more comfortable and slightly more breathable. Tri-blends tend to be a little pricier but they are by far more comfortable than cotton and you get a vintage, fitted look and extreme durability.